Your agent writes secret.txt with the placeholder, and the tokenizing proxy replaces it with the token, then the agent reads secret.txt
Couldn't it then just publish the mock in a public place... it would get replaced by the real secret.? How is this prevented
Maybe the tokenizing proxy could work both ways? If the agent tries to read secret.txt, it gets back the placeholder.
It only replaces the token in the HTTP header that is sent to the server. Whatever you wrote in your files isn't touched by the proxy.